The causes of ozone layer degradation are examined by statistical analysis of time series of total ozone content from European ozonometric stations from 1958 to 1991, including the influence of the Azores anticyclone. It is shown that the dynamic factors must be taken into account in quantitative estimations of the total ozone content trends because the long-period changes of the factors affect the trends significantly. The January pressure at the center of the Azores anticyclone can be used as a dynamic factor determining permanent changes of the annual mean total ozone content over Europe (at least as the first approximation).